The article “Effect of Electron Traps on Scintillation of Praseodymium Activated Lu3Al5O12” by W. Drozdowski et al., IEEE Transactions on Nuclear Science, volume 56, number 1, pages 320 to 327 (2009) discloses a measurement of x-ray excited emission spectra and Cs-137 gamma-ray pulse height spectra in a range from 78 to 600 K and of thermoluminescence glow curves.
The article “Non-resonant X-ray/laser interaction spectroscopy as a method for assessing charge competition, trapping and luminescence efficiency in wide band-gap materials” by N. R. J. Poolton et al., Journal of Luminescence, volume 130, pages 1404 to 1414 (2010) discloses an apparatus for non-resonant x-ray/laser interaction spectroscopy, which comprises an x-ray source with a shutter and a pulsed laser diode module.
In positron emission tomography (PET) imaging systems scintillator materials are used, in order to emit visible or ultraviolet (UV) light after excitation by 511 keV gamma quanta with high photon gain, good energy resolution, fast signal decay and fast signal rise. Known scintillator materials, which generally fulfill these properties, are, for instance, bismuth germanate (BGO), cadmium tungstate (CWO), lutetium orthosilicate (LSO), LSO modified by 10 percent of yttrium instead of lutetium (LYSO) and gadolinium based garnets like lutetium gadolinium gallium aluminum garnets (LGGAG).
The actual performance of these scintillator materials depends strongly on the number of electronic defects, i.e. the number of traps, in the scintillator materials. These traps may be caused by, for instance, contamination of raw materials, inexact stoichiometry, loss of oxygen or other parts of the respective compounds like gallium, resulting in vacancies, anti-site defects et cetera. During a PET imaging procedure several thousands of electrons and holes may be generated after absorption of the 511 keV gamma quanta, wherein a recombination of these charge carriers at luminescent sites of the scintillator material should ideally result in a high number of optical photons, which are emitted within a short time with a pulse shape defined by a rise time in the range of, for instance, 100 ps to 2 ns and a decay time of, for example, a few tens of nanoseconds and which are detected by an optical detector of the PET imaging system within an integration time of, for instance, a few hundred nanoseconds. However, in the presence of electronic defects the charge carriers will be partly trapped at these defects, giving rise to delayed luminescence or resulting in a non-luminescent recombination. This can result in a change in photon gain, a lower energy resolution, a lower timing performance, i.e. an increased coincidence resolving time (CRT), and a deterioration of image quality. The scintillator material should therefore be characterized with respect to its expected performance and only scintillator materials, which have a desired performance, should be selected and used for manufacturing a PET detection device.
A known characterization technique is based on a measurement of x-ray afterglow. The scintillator material is irradiated with an x-ray pulse having a relative high radiation dose of up to 1 Gy and then an afterglow signal is measured, wherein the scintillator material is characterized based on the measured afterglow signal. However, this technique requires a measurement of the afterglow signal over a very large dynamic range, which might be about six orders of magnitude, and over a very long time, i.e. up to days, which can render the measurement very laborious.